1. Field of the Invention
This invention relates to an ink jet recording apparatus for discharging ink to a recording medium to thereby effect recording of characters, images and the like, and in particular to an ink jet recording apparatus having a temperature compensating function.
2. Related Background Art
An ink jet recording apparatus of the type in which the driving voltage of an ink jet head is varied to vary the amount of ink discharge, whereby the recording dot diameter is varied to express half-tone, has already been proposed.
However, the apparatus of this type has the temperature characteristic that the value of a property of the recording liquid, i.e., the viscosity or surface tension of ink, is greatly varied by temperature and the discharge of the head itself is varied by temperature. Accordingly, the relation between the optical density on recording paper after recording and the voltage applied to the head is varied by temperature as generally shown in FIG. 2 of the accompanying drawings. That is, as temperature becomes higher, the optical density becomes higher even for the same voltage applied to the head.
Therefore, if printing is effected at 10.degree. C. by the use, for example, of the same density-voltage data used at 25.degree. C. as has heretofore been done, there will occur an inconvenience that printing density becomes low and ink is not discharged in the low voltage area, and if printing is effected at 40.degree. C. by the use of the same density-voltage data used at 25.degree. C., there will occur various inconveniences, including the variation in the density in the image representation range, that the amount of ink discharge becomes great and print becomes too dark and the recording paper becomes unable to absorb ink and the ink oozes.
On the other hand, as the conventional temperature compensating method, there is a method of using a heater or the like to keep the value of the property of ink constant as disclosed in Japanese Patent Applications Laid-Open Nos. 188363/1982 and 188364/1982, and a method of varying the voltage applied to the had in conformity with temperature as disclosed in Japanese Patent Applications Laid-Open Nos. 27210/1980 and 14759/1983. However, the former method suffers from disadvantages such as bulkiness of the apparatus, increased capacity of the power source, which in turn leads to increased manufacturing cost, and unsatisfactory printing resulting from the production of soluble gas of ink caused by rapid heating.
The latter method is effective only with respect to binary images, and if the amount of ink discharge is to be varied by this method to thereby express half-tone, the circuit construction will become very complicated for non-linear variations in various characteristics, and this has led to higher cost and difficulty in putting this method into practical use. This latter method has further suffered from a problem that during low temperatures, increased viscosity of ink causes the return of meniscus after discharge to be delayed, which results in a reduced response frequency leading to the necessity of compensating for the frequency characteristic of the head at each temperature.